1. Field of the Invention
The present invention relates to a magnetic recording medium used for high-density magnetic recording, a method and an apparatus for manufacturing the magnetic recording medium, and a magnetic recording apparatus using the magnetic recording medium.
2. Description of the Related Art
Magnetic recording apparatuses such as hard disk drives (HDD) are increasingly used widely with widespread use of personal computers. In recent years, with the advent of the Internet and DVDs storing high-definition image information, the amount of information to be processed is increasing rapidly and demands for larger capacities are growing. Further, small-sized HDDs are mounted in mobile devices such as mobile phones, car navigation systems, and MP3 players, raising expectations for still higher density. Such a situation can be considered to have been brought about by rapid improvement in recording density of HDDs. In HDDs, recording density is improved by forming still smaller magnetic recording marks. To form still smaller marks, a still smaller write head, a read head capable of detecting a still lower magnetic field, and a magnetic recording medium capable of writing still smaller marks with stability are needed.
Conventionally, magnetic grains constituting a magnetic recording layer deposited by sputtering have been made finer to form still smaller marks for magnetic recording medium. However, due to degraded thermal stability of fine magnetic grains or a so-called thermal fluctuation problem, making magnetic grains finer is becoming more and more difficult. Thermal stability of magnetic material itself may be improved to solve the thermal fluctuation problem, but this also increases resistance to a recording magnetic field, making a more intense magnetic field necessary for recording. However, magnetic field strength obtained from a write head is currently approaching its limit.
Against the background described above, a patterned medium having a structure significantly different from that of a conventional magnetic recording medium has been proposed. In the patterned medium, recording cells, which are the minimum recording units, are arrayed on tracks by lithography. While data is written onto an aggregation of several tens to several hundreds of magnetic grains even for a minimum recording mark in a conventional magnetic recording layer deposited by sputtering, the size of magnetic grains can be increased to that of recording cells formed by lithography in the patterned medium. Therefore, the patterned medium can fundamentally solve the thermal fluctuation problem resulting from finer magnetic grains.
Because the position of each recording cell to which data is written is fixed in a patterned medium, it becomes difficult to write data of a desired polarity if a recording magnetic field is not generated at synchronized timing when a write pole comes immediately above the recording cell. Particularly when recording density has improved and linear recording density has increased, precision required for write timing becomes higher, which presents a formidable challenge in making a magnetic recording apparatus using a patterned medium denser. In order to solve the problem, a method of forming phase information of recording cells on a medium to synchronize timing has been proposed. See, for example, JP-A 2006-164349 (KOKAI) and JP-A 2003-157507 (KOKAI).
Whether data is written when a recording magnetic field is applied to a certain recording cell needs to be uniquely determined depending on the position where the recording magnetic field is applied. When recording cells are ideally magnetically homogeneous, if a recording magnetic field is applied upstream from the central position of a recording cell in the traveling direction of a head, data is written into the recording cell. If a recording magnetic field is applied downstream from the central position, no data is written into the recording cell. That is, a timing margin for writing data into a certain recording cell will be a duration in which the head is positioned between the central position of the recording cell and that of an adjacent recording cell. However, magnetic characteristics of recording cells cannot be actually homogeneous inside the recording cells. Therefore, in a certain recording cell, there is a point that determines whether data is written into a position deviating from the central position of the recording cell. Such dispersion of recording points reduces the timing margin for applying a recording magnetic field. That is, if a recording magnetic field is applied within the range of dispersion thereof, whether data is written into a certain dot will not be determined. Therefore, a recording magnetic field needs to be applied in a narrow range excluding the width thereof so that timing to apply a recording magnetic field will have to be controlled more precisely. Such dispersion can probably be caused by heterogeneity of composition when magnetic recording films are deposited, distribution of impurities, heterogeneity of damage when recording cells are processed and the like.